Photodetector

ABSTRACT

A photodetector device comprising n-type and p-type light absorbing regions arranged to form a pn-junction and n+ and p+ contact regions connected to respective contacts. The light absorbing regions and the contact regions are arranged in a sequence n+ p n p+ so that, after a voltage applied between the n+ and p+ contacts is switched from a reverse bias to a forward bias, electrons and holes which are generated in the light absorbing regions in response to photon absorption drift towards the p+ and n+ contact regions respectively, which causes current to start to flow between the contacts after a time delay which is inversely proportional to the incident light intensity.

FIELD OF THE INVENTION

The present disclosure relates to a photodetector.

BACKGROUND

A traditional pn or pin photodetector is operated by being held at aconstant reverse bias voltage. Incident photons are absorbed in a lightabsorbing region to generate electron-hole pairs which are swept to thecontacts, so that the magnitude of the photocurrent is proportional tothe intensity of the light incident on the photodiode.

A non-traditional type of photodetector is disclosed in US 2012/313155A1 which, operates using pulsed voltages that are switched from reversebias to forward bias. Switching to forward bias induces a photocurrentto flow across the device structure. However, the onset of the flow ofphotocurrent is not instantaneous, but rather occurs after a time delayfrom the onset of the light incidence. This time delay is referred to asthe triggering time. The triggering time is proportional to the inverseof the light intensity, so triggering time is used as the measure of theintensity of the incident light.

FIG. 1A and FIG. 1B are schematic representations in section and planview respectively of a photodetector as disclosed in US 2012/313155 A1.The growth direction, i.e. orthogonal to the plane of the wafer, ismarked as the z-direction. First and second gates, Gate 1 and Gate 2,extend in the y-direction, and the direction orthogonal to the gates, inwhich the electrons and holes are swept out, is the x-direction. Thesection AA of FIG. 1A is in the xz-plane as indicated in FIG. 1B. Gate 1and Gate 2 are arranged either side of a light absorbing region whichforms part of a body region. The body region may be an intrinsic or adoped semiconductor such as silicon or germanium suitable for absorbingincoming photons of the wavelength range to be detected. Highly doped n+and p+ regions are arranged either side of the body region beyond thegates and serve as outputs for reading out the photocurrent. The layersof the photodetector are epitaxially fabricated on asemiconductor-on-insulator (SOI) substrate. The gates are made of aconductive material (metal or semiconductor). The gates are spaced fromthe body region via an insulator or dielectric material (silicon oxideor silicon nitride). The photodetector is operated with the followingbias voltages. A negative voltage VG1 is applied to Gate 1 (for example,−2V), a negative or zero voltage V1 is applied to the n+ region, apositive voltage VG2 applied to Gate 2 (for example, 2V) and a positivevoltage V2 (for example, 1V) is applied to the p+ region. The triggeringtime of the photodetector is a function of the electric field in thebody region and his hence tunable by adjusting the gate voltages. Underthese bias conditions, photons incident onto the light absorbing regionbetween the gates, e.g. from a fiber optic device, are absorbed andthereby generate electron-hole pairs which is then swept out by theelectric field induced by the bias voltages and so detected as currentflowing between the n+ and p+ regions.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the disclosure, there is provided aphotodetector device comprising: first and second light absorbingregions composed of semiconductor material and which are dopedrespectively n-type and p-type, the light absorbing regions beingconfigured to generate pairs of electrons and holes in response toabsorption of photons when light is incident on the device; first andsecond contact regions composed of semiconductor material and which aredoped respectively p-type and n-type, wherein the contact regions havehigher doping concentrations than the light absorbing regions, and arelabelled as p+ and n+ respectively; and first and second contactsconnected to the first and second contact regions respectively. Then-type and p-type light absorbing regions and the n+ and p+ contactregions are arranged in a sequence n+ p n p+ so that, after a voltageapplied between the n+ and p+ contacts is switched from a reverse biasto a forward bias, electrons and holes which are generated in the lightabsorbing regions in response to photon absorption drift towards the p+and n+ contact regions respectively, which causes current to start toflow between the contacts after a time delay which is inverselyproportional to the incident light intensity.

In some embodiments, the first and second light absorbing regions arearranged side-by-side in relation to a substrate. The first and secondcontact regions may be arranged laterally spaced apart either side ofthe first and second light absorbing regions.

In some embodiments, the first and second light absorbing regions areformed as respective epitaxial layers arranged on top of one another inrelation to a substrate. The device may further comprise insulatingtrenches extending vertically through the structure to define aphotodetector array of pixels. The trenches extend through the lightabsorbing regions and at least one of the contact regions so as tosubdivide the photodetector into an array of pixels that areindependently contacted, and so can be individually read out by suitableelectronics. The photodetector may further comprise a semiconductorcircuit layer, for example in silicon CMOS technology, arranged on theepitaxial layer that forms one of the first and second contact regions,the circuit layer comprising an array of read out sensors for thephotodetector's pixel array electrically connected to the pixels withvias. In a refinement of the photodetector array, the pixels are eachsubdivided into an array of sub-pixels by further insulating trenches.The further trenches for each pixel are disposed laterally inside thepixel-defining insulating trenches. The further trenches extendvertically through one of the contact regions and at least one of thelight absorbing regions, but not as far as the other of the contactregions, so that the sub-pixels of any one pixel remain commonlycontacted, so the sub-pixels of any one group are biased and read out asa single pixel unit.

In some embodiments, the first and second light absorbing regions arearranged such that one of the first and second light absorbing regionsis formed as an epitaxial layer on a substrate, or integrally with thesubstrate, and the other of the first and second light absorbing regionsis formed as an embedded region within the epitaxial layer.

In some embodiments, the first and second light absorbing regions arearranged such that one of the first and second light absorbing regionsis formed in a first part as an epitaxial layer on a substrate, orintegrally with the substrate, and in a second part as an embeddedregion within the epitaxial layer or the substrate, and wherein theother of the first and second light absorbing regions is formed as afurther embedded region within the epitaxial layer. The embedded regionand the further embedded region may be separated laterally by a portionof the epitaxial layer or the substrate. One of the first and secondcontact regions may be formed as a still further embedded region withinthe embedded region of the first or second light absorbing regionsrespectively. The first and second contact regions may be formed asrespective still further embedded regions within the embedded regions ofthe first and second light absorbing regions respectively.

In some embodiments, one of the first and second contact regions isformed as an epitaxial layer on the substrate and at least one of thefirst and second light absorbing regions is formed at least in part as afurther epitaxial layer on the epitaxial layer of said one of the firstand second contact regions.

In some embodiments, at least one of the first and second contactregions is formed as an embedded region within an epitaxial layer whichforms at least a part of the first and second light absorbing regionsrespectively.

In some embodiments, the first and second contact regions are formed asrespective laterally spaced apart first and second embedded regionswithin an epitaxial layer which forms at least a part of the first andsecond light absorbing regions respectively.

In some embodiments, wherein one of the first and second contact regionsis formed as laterally spaced apart first and second embedded regionsformed within an epitaxial layer which forms at least a part of one ofthe first and second light absorbing regions respectively.

It will be understood that the semiconductor material from which thelight absorbing regions are made is or are selected having regard to itsor their band gaps in order that interband absorption of photons occursover a desired energy range as required by the photodetector to fulfilla specification. The semiconductor material of the two light absorbingregions may be the same so that the pn-junction between the p-type andn-type regions is a homojunction, or two different semiconductormaterials could be chosen so that the pn-junction is a heterojunction.In the case of a heterojunction the two different materials may be inthe same materials' system and so be capable of forming alloys with eachother, e.g. the SiGeC materials' system, or the GaAlInAsP materials'system.

According to another aspect of the disclosure, there is provided amethod of manufacturing a photodetector device, the method comprising:fabricating first and second light absorbing regions composed ofsemiconductor material and which are doped respectively n-type andp-type, the light absorbing regions being configured to generate pairsof electrons and holes in response to absorption of photons when lightis incident on the device; fabricating first and second contact regionscomposed of semiconductor material and which are doped respectivelyp-type and n-type, wherein the contact regions have higher dopingconcentrations than the light absorbing regions, and are labelled as p+and n+ respectively; and providing first and second contacts connectedto the first and second contact regions respectively. The n-type andp-type light absorbing regions and the n+ and p+ contact regions arearranged in a sequence n+ p n p+ so that, after a voltage appliedbetween the n+ and p+ contacts is switched from a reverse bias to aforward bias, electrons and holes which are generated in the lightabsorbing regions in response to photon absorption drift towards the p+and n+ contact regions respectively, which causes current to start toflow between the contacts after a time delay which is inverselyproportional to the incident light intensity.

In the above method, the first and second light absorbing regions may befabricated as respective epitaxial layers arranged on top of one anotherin relation to a substrate. Moreover, the method may further comprisefabricating insulating trenches extending vertically through the lightabsorbing regions and at least one of the contact regions so as tosubdivide the photodetector into an array of pixels that areindependently contacted. Further insulating trenches may be providedsuch that each pixel is subdivided into an array of sub-pixels by thefurther insulating trenches, which for each pixel are disposed laterallyinside the pixel-defining insulating trenches and which extendvertically through one of the contact regions and at least one of thelight absorbing regions, but not as far as the other of the contactregions, so that the sub-pixels of any one pixel remain commonlycontacted.

According to a further aspect of the disclosure, there is provided amethod of operating a photodetector device as specified above. Themethod comprises operating the photodetector device by repeatedly:applying a voltage to reverse bias the n+ and p+ contacts; switching thereverse bias voltage to a forward bias voltage so that after saidswitching electrons and holes which are generated in the light absorbingregions in response to photon absorption drift towards the p+ and n+contact regions respectively; and sensing for onset of current flowbetween the first and second contacts. The time delay between saidswitching and said onset is measured, the time delay being inverselyproportional to the incident light intensity. This reverse-to-forwardbiasing sequence is then repeated.

In summary, we propose a photodetector device and corresponding methodof manufacture for a photodetector comprising n-type and p-type lightabsorbing regions arranged to form a pn-junction and n+ and p+ contactregions connected to respective contacts. The light absorbing regionsand the contact regions are arranged in a sequence n+ p n p+ so that,after a voltage applied between the n+ and p+ contacts is switched froma reverse bias to a forward bias, electrons and holes which aregenerated in the light absorbing regions in response to photonabsorption drift towards the p+ and n+ contact regions respectively,which causes current to start to flow between the contacts after a timedelay which is inversely proportional to the incident light intensity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will further be described by wayof example only with reference to exemplary embodiments illustrated inthe figures.

FIGS. 1A and 1B are schematic representations in section and plan viewrespectively of a prior art photodetector as disclosed in US 2012/313155A1.

FIGS. 2A and 2B are schematic representations in section and plan viewrespectively of a photodetector according to a first embodiment of theinvention.

FIGS. 3A, 3B, and 3C are energy band diagrams showing a photodetectoraccording to the first embodiment with the photodetector respectively ina reversed-biased state, in a forward-biased conducting state and aforward-biased non-conducting state.

FIG. 4 is a graph of output current as a function of bias voltage forthe photodetector according to the first embodiment of FIGS. 2A and 2Bwith (“ON”) and without (“OFF”) incident light, i.e. the forward-biasedconducting and non-conducting states of FIGS. 3C and 3B respectively.

FIGS. 5A and 5B show oscilloscope scope screen shots of applied voltageV_(d) and output current I without and with light, respectively;

FIG. 6 is a graph plotting reciprocal triggering time as a function ofabsorbed light power.

FIG. 7 is a schematic section of a photodetector according to a secondembodiment of the invention.

FIG. 8 is a schematic section of a photodetector according to a thirdembodiment of the invention.

FIG. 9 is a schematic section of a photodetector according to a fourthembodiment of the invention.

FIG. 10 is a schematic section of a photodetector according to a fifthembodiment of the invention.

FIG. 11 is a schematic section of a photodetector according to a sixthembodiment of the invention.

FIG. 12 is a schematic section of a photodetector according to a seventhembodiment of the invention.

FIG. 13 is a schematic section of a photodetector according to an eighthembodiment of the invention.

FIG. 14 is a schematic section of a photodetector according to a ninthembodiment of the invention.

FIG. 15 is a schematic section of a photodetector according to a tenthembodiment of the invention.

FIG. 16 illustrates a detector array comprising a two-dimensional (2D)array of light sensors, each light sensor of the array being aphotodetector as described above.

FIG. 17 shows a photodetector according to any one of the aboveembodiments in operation as a high-speed optoelectronic converter.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, specific details are set forth in order to provide abetter understanding of the present disclosure. It will be apparent toone skilled in the art that the present disclosure may be practiced inother embodiments that depart from these specific details.

FIGS. 2A and 2B are schematic representations in section and plan viewrespectively of a photodetector according to a first embodiment of theinvention.

The growth direction, i.e. orthogonal to the plane of the wafer, ismarked as the z-direction. First and second contact regions, which aresemiconductor regions doped n+ and p+ respectively, extend in they-direction. The section AA of FIG. 2A is in the xz-plane as indicatedin FIG. 2B. The contact regions are arranged either side of a bodyregion which is formed in two parts, namely a p-doped semiconductorsub-region adjacent the n+ contact region, and an n-doped semiconductorregion adjacent the p+ contact region, where the n-type and p-typesubregions have a common interface where they meet. In the x-direction,the structure thus has the sequence in the x-direction of: n+-p-n-p+.The body sub-regions may be made of a suitable semiconductor materialsuch as silicon or germanium or silicon carbide, or suitable alloythereof, where the band gap is chosen to be suitable for absorbingincoming photons of the wavelength range to be detected. Highly doped n+and p+ regions are arranged either side of the body region beyond thegates and serve as outputs for reading out the photocurrent. The layersof the photodetector are epitaxially fabricated on asemiconductor-on-insulator (SOI) substrate.

The photodetector can be operated with the following bias voltages. Anegative or zero voltage V1 (for example, 0 or −1V) is applied to the n+region and a positive voltage V2 (for example, +1V) is applied to the p+region. The n- and p-type sub-regions of the body region are unbiased.The triggering time of the photodetector is a function of the electricfield in the body region and his hence tunable by adjusting the biasvoltages V1 and V2. Under these bias conditions, photons incident ontothe light absorbing region, e.g. from a fiber optic device, are absorbedand thereby generate electron-hole pairs which is then swept out by theelectric field induced by the bias voltages and so detected as currentflowing between the n+ and p+ regions.

FIGS. 3A, 3B, and 3C are energy band diagrams along the x-direction.FIG. 3A shows the photodetector in reverse bias. FIGS. 3B and 3C bothshow the photodetector in forward bias with a bias voltage V2−V1. FIG.3B shows a condition when no light is incident, resulting in thestructure being in a non-conducting state, and FIG. 3C shows a conditionwhen light is incident, resulting in the structure being in a conductingstate. In forward bias, when the sensor does not detect any light,little to no current flows between the p+ and n+ regions due to thebarrier provided or caused by the bias voltage V2−V1. However, when thesensor detects light, the incident photons are absorbed to generateelectron-hole pairs and the sensor changes to a conducting state.Namely, under the electric field generated by the bias voltage, theholes move towards the n+ contact region, and the electrons move towardsthe p+ contact region. The migrated holes accumulate in the part of thep-region adjacent the n+ contact region and induce a lowering of thepotential barrier to electron movement and electron current flow fromthe n+ region. Similarly, the migrated electrons accumulate in the partof the n-region adjacent the p+ contact region and induce a lowering ofthe hole barrier and hole current flowing from the p+ region. In itsconductive state, the sensor provides a large internal current gain. Inaddition, a positive feedback mechanism accelerates accumulation ofexcess positive and negative carriers adjacent the respective n+ and p+contact regions, which, in turn, reduce the potential barriers relatedcorresponding to such regions and causes a current to flow between thep+ and n+ regions of the light sensor and an output current upondetecting or in response to the incident light.

FIG. 4 is a schematic graph showing output current of the photodetectoras a function of bias voltage V2−V1 between the n+ and p+ contactregions when incident light is detected (“ON”), and is not, detected(“OFF”), i.e. the conducting and non-conducting states of FIGS. 3C and3B respectively. It is noted that above a threshold bias voltage Vth,the output current in the conducting state is more or less static withvarying bias voltage, which is a preferred operating regime given thatincident light intensity is measured by triggering time, notphotocurrent magnitude.

FIGS. 5A and 5B shows an oscilloscope screen shot of applied voltageVd=V2−V1 and output current I without and with light, respectively.Triggering time t decreases with increase in light intensity. FIG. 5Ashows a triggering time of t0=5.5 μs with no light. FIG. 5B shows atriggering time of t1=1.5 μs with light at an absorbed power of 35 nW.Switching from a low current state to a high current state occurs veryabruptly, which is favorable for precise measurements of delay time. Theoutput current of 0.8 mA is more than four orders higher than an outputcurrent that could be achieved with a conventional photodiode at anabsorbed power of 35 nW.

FIG. 6 is a graph plotting reciprocal triggering time as a function ofabsorbed light power. As can be seen there is a linear relationshipbetween the inverse of triggering time and absorbed light power.

FIG. 7 is a schematic section of a vertical photodetector according to asecond embodiment of the invention. By vertical, we mean that the layersare epitaxially formed in the xy-plane, which is the plane of thesubstrate, so the layer sequence is in the z-direction. The structure issubdivided into individual pixels, in a one-dimensional array of rows,or a two-dimensional array of rows and columns by insulating trenchesfilled with dielectric material that electrically isolate adjacentpixels from each other. The dielectric material may be material that isdeposited after etching, or material that is generated by an oxidizationprocess after etching, for example. Instead of filling the trenches withdielectric material, they could be left unfilled or only be partlyfilled by a thin layer of oxide or other insulating material coating thesides of the trenches. The insulating trenches thus extend verticallythrough the light absorbing regions and at least one of the contactregions so as to subdivide the photodetector into an array of pixelsthat are independently contacted.

Semiconductor layers are deposited on a suitable substrate in thesequence p+ n p n+ as illustrated, or in the reverse sequence. Thedoping of each layer may be achieved at the time of deposition, orthrough post-deposition processes, such as ion implantation, or acombination of both, as desired. The n-type and p-type layers form thedetector's light absorbing regions and the n+ and p+ layers its contactregions. The n-type and p-type layers have an interface which forms apn-junction. The n-type and p-type layers have band gaps suitable forabsorbing photons of a specified wavelength (energy) range and generatepairs of electrons and holes that drift towards the p+ and n+ layersrespectively when the pn-junction is under a forward bias. Anelectron-hole pair generated by absorption of a photon in the p-layer(as schematically illustrated) or in the n-layer while the device isunder forward bias are separated by the forward-bias induced appliedelectric field with holes drifting towards the n+ layer and electronstowards the p+ layer. The substrate is not shown, but a suitablesubstrate, such as a p+ substrate for ohmically contacting the pixels ofthe p+ layer, may be provided. When the structure is switched from areverse bias to a forward bias in respect of the pn-junction,electron-hole pairs generated by photon absorption initiate a currentflow between the contacts once a sufficient number of electrons andholes have drifted to cause the barrier to be decreased sufficiently.There is thus a time delay from the reverse-to-forward bias switchingevent to the onset of current flow which is inversely proportional tothe incident light intensity.

The photodetector is operated by repeated cycles of switching fromreverse to forward bias. Namely, operation proceeds by applying avoltage to reverse bias the n+ and p+ contacts; switching the reversebias voltage to a forward bias voltage. After the switching, electronsand holes which are generated in the light absorbing regions in responseto photon absorption drift towards the p+ and n+ contact regionsrespectively. The device then senses for onset of current flow betweenthe first and second contacts. The time delay between said switching andsaid onset is measured, the time delay being inversely proportional tothe incident light intensity. This reverse-to-forward biasing sequenceis then repeated. The repeat cycling of the drive and read out may beperiodic or aperiodic. In the periodic case, the duration of the forwardbias and reverse bias segments are fixed. In the aperiodic case, thereverse bias segment is of fixed duration, but the forward bias durationis varied responsive to the incident light intensity within a timewindow set between a minimum value and a maximum value. After onset ofcurrent has occurred, and the time delay has been measured, the forwardbias segment of the cycle can be terminated. The forward bias durationwill then have the maximum value when there is no incident light, sincethere will be no onset of current, and have the minimum value when theincident light intensity is high, since the time delay will be shorterthan the minimum value, but have an intermediate value when the incidentlight intensity is such that the time delay for the onset of current iswithin the window.

FIG. 8 is a schematic section of a vertical photodetector according to athird embodiment of the invention, which will be largely understood fromthe previous discussion of FIG. 7. In the third embodiment, each pixelconsists of a group of subpixels. As in the second embodiment, eachpixel is defined by a dielectric material trench extending through thewhole structure, i.e. through the n+ p n p+ layers. The subpixels of agiven pixel are divided from each other by dielectric material trenches,but ones which extend partly, but not wholly, through the structure,namely at least through the top contact layer (here n+) and at leastpartway through the upper one of the light absorbing layers (here p) andpossibly also partway through the lower one of the light absorbinglayers (here n). Each pixel is thus subdivided into a one- ortwo-dimensional array of sub-pixels by further insulating trenches whichfor each pixel are laterally inside the pixel-defining insulatingtrenches and which extend vertically through one of the contact regionsand at least one of the light absorbing regions, but not as far as theother of the contact regions, so that the sub-pixels of any one pixelremain commonly contacted. The subpixel structure may serve to reduceinternal capacitance and thereby provide better sensitivity.

FIG. 9 is a schematic section of a vertical photodetector according to afourth embodiment of the invention. The photodetector structure shown inFIG. 7 is combined with a semiconductor circuit layer arranged on theupper contact region. The circuit layer comprises an array of read outsensors for the photodetector's pixel array with the sensor-to-pixelconnections being implemented with vias. In particular, the circuitlayer may be CMOS circuit layer which makes it electrical connections tothe pixels with through-silicon vias (TSVs). Bias voltages can then beapplied to the n+ and p+ contact regions through the TSVs. Moreover,signal current induced by incident light can be detected on a per pixelbasis through the TSV connections. The CMOS circuit layer is shownarranged on the n+ contact layer, but alternatively it could be arrangedon the p+ contact layer. The structure of FIG. 7 may also be combinedwith a CMOS circuit layer in similar fashion.

FIG. 10 is a schematic section of a photodetector according to a fifthembodiment of the invention. This embodiment may be understood as avariant of the FIG. 7 embodiment in that the epitaxial layer structurefrom bottom to top comprises a p+ contact layer, an n-type layer and ap-type layer, where the n- and p-type layers form the light absorbingregions. However, instead of forming the top contact as an epitaxiallayer, the top contact is formed with one or more embedded regionswithin the p-type epitaxial layer. Two adjacent embedded regions areshown, wherein an arrangement of adjacent embedded regions may serve todefine a pixel. Alternatively, dielectric material trenches may be usedas described in other embodiments. It is also noted that light may beincident from the cleaved side surface in some embodiments asschematically illustrated. The substrate is not shown, but a suitablesubstrate, such as a p+ substrate for ohmically contacting the p+ layer,may be provided. It will also be understood that the reverse structurecould be implemented, i.e. as illustrated but with n+→p+, p→n, n→p, andp+→n+.

FIG. 11 is a schematic section of a photodetector according to a sixthembodiment of the invention. A horizontal n+p n p+ structure is usedwhich is somewhat similar conceptually to that of FIGS. 2A/2B. A p-typelayer is deposited on a substrate. In the p-type layer, an embeddedn-type region is formed, and within the n-type region a p+ contactregion is formed. Laterally offset from the n-type region, an n+ contactregion is formed in the p-type layer. It will also be understood thatthe reverse structure could be implemented. Moreover, the substrate isnot an electrically active part of the device, so may be for examplen-type, p-type, semi-insulating semiconductor, sapphire or an insulator,as desired.

FIG. 12 is a schematic section of a photodetector according to a seventhembodiment of the invention. A p-type or n-type substrate is provided.In a surface of the substrate, an embedded n-type region is formed, andwithin the n-type region a p+ contact region is formed. Laterally offsetfrom the n-type region, a p-type embedded region is formed and withinthat an n+ contact region.

FIG. 13 is a schematic section of a photodetector according to an eighthembodiment of the invention. This is a variant of the FIG. 12 embodimentin which the role of the substrate in FIG. 12 is taken by an epitaxiallayer. The substrate is therefore not an electrically active part of thedevice, so may be for example n-type, p-type, semi-insulatingsemiconductor, sapphire or an insulator, as desired.

FIG. 14 is a schematic section of a photodetector according to a ninthembodiment of the invention. A p-type layer is deposited on a substrate.In the p-type layer, an embedded n-type region is formed, and within then-type region a p-type region, and within the p-type region a p+ contactregion. Laterally offset from the n-type region, an n+ contact region isformed in the p-type layer. It will also be understood that the reversestructure could be implemented. Moreover, the substrate is not anelectrically active part of the device, so may be for example n-type,p-type, semi-insulating semiconductor, sapphire or an insulator, asdesired. In this device structure the active pn-junction in respect ofthe time-delayed light-induced current is the one pointed to in thediagram.

FIG. 15 is a schematic section of a photodetector according to a tenthembodiment of the invention. A p-type layer is deposited on a p+substrate. In the p-type layer, an embedded n-type region is formed, andwithin the n-type region a p-type region, and within the p-type region an+ contact region. Laterally offset from the n-type region, a p+ contactregion is formed in the p-type layer. It will also be understood thatthe reverse structure could be implemented. In this device structure theactive pn-junction in respect of the time-delayed light-induced currentis the one pointed to in the diagram.

FIG. 16 illustrates a detector array comprising a two-dimensional (2D)array of light sensors, each light sensor of the array being aphotodetector as described above. The detector array may include, inaddition to the array of sensors, control circuitry to manage theacquisition, capture and/or sensing operations of the light sensors ofthe array. For example, the control circuitry (which may be integratedon the same substrate as the sensors) may control or enable/disable thesensors in a manner so that data acquisition or sensing correlates tothe data rate of the transmission; the detector array may be coupled toa plurality of fiber optic output devices wherein each fiber opticdevice is associated with one of the sensors, or a group of the sensors.The sensors may be configured and/or arranged in any array architectureas well as in conjunction with any type of integrated circuitry.Further, any manufacturing technique may be employed to fabricate thearray.

FIG. 17 shows a photodetector according to any one of the aboveembodiments coupled to an optional current amplifier. The photodetectoris illustrated in operation as a high-speed optoelectronic converteroperable to convert an optical pulse train, or other more complexsignal, into an equivalent electrical pulse train, or other more complexsignal. The schematically illustrated pulse train above shows theoptical signal transmitted through the optical fiber, and theschematically illustrated pulse train below shows the electrical signaloutput by the photodetector that has received the optical signal asinput. Due to the large output signal, the photodetector does notrequire an amplifier and can be directly connected to digital circuits.Not requiring an amplifier is advantageous, since an amplifier is anoise source.

It should be noted that the term “circuit” may mean, among other things,a single component or a multiplicity of components (whether inintegrated circuit form or otherwise), which are active and/or passive,and which are coupled together to provide or perform a desired function.The term “circuitry” may mean, among other things, a circuit (whetherintegrated or otherwise), a group of such circuits, one or moreprocessors, one or more state machines, one or more processorsimplementing software, one or more gate arrays, programmable gate arraysand/or field programmable gate arrays, or a combination of one or morecircuits (whether integrated or otherwise), one or more state machines,one or more processors, one or more processors implementing software,one or more gate arrays, programmable gate arrays and/or fieldprogrammable gate arrays. The term “data” may mean, among other things,a current or voltage signal(s) whether in an analog or a digital form,which may be a single bit (or the like) or multiple bits (or the like).

It should be further noted that the various circuits and circuitrydisclosed herein may be described using computer aided design tools andexpressed (or represented), as data and/or instructions embodied invarious computer-readable media, for example, in terms of theirbehavioral, register transfer, logic component, transistor, layoutgeometries, and/or other characteristics. Formats of files and otherobjects in which such circuit expressions may be implemented include,but are not limited to, formats supporting behavioral languages such asC, Verilog, and HLDL, formats supporting register level descriptionlanguages like RTL, and formats supporting geometry descriptionlanguages such as GDSII, GDSIII, GDSIV, CIF, MEBES and any othersuitable formats and languages. Computer-readable media in which suchformatted data and/or instructions may be embodied include, but are notlimited to, non-volatile storage media in various forms (e.g., optical,magnetic or semiconductor storage media) and carrier waves that may beused to transfer such formatted data and/or instructions throughwireless, optical, or wired signaling media or any combination thereof.Examples of transfers of such formatted data and/or instructions bycarrier waves include, but are not limited to, transfers (uploads,downloads, e-mail, etc.) over the Internet and/or other computernetworks via one or more data transfer protocols (e.g., HTTP, FTP, SMTP,etc.). The present inventions are also directed to such representationof the circuitry described herein, and/or techniques implementedthereby, and, as such, are intended to fall within the scope of thepresent inventions.

Indeed, when received within a computer system via one or morecomputer-readable media, such data and/or instruction-based expressionsof the above described circuits may be processed by a processing entity(e.g., one or more processors) within the computer system in conjunctionwith execution of one or more other computer programs including, withoutlimitation, net-list generation programs, place and route programs andthe like, to generate a representation or image of a physicalmanifestation of such circuits. Such representation or image maythereafter be used in device fabrication, for example, by enablinggeneration of one or more masks that are used to form various componentsof the circuits in a device fabrication process.

Moreover, the various circuits and circuitry, as well as techniques,disclosed herein may be represented via simulations and simulationinstruction-based expressions using computer aided design, simulationand/or testing tools. The simulation of the circuitry of the presentinventions, including the photodetector and/or techniques implementedthereby, may be implemented by a computer system wherein characteristicsand operations of such circuitry, and techniques implemented thereby,are simulated, imitated, replicated, analyzed and/or predicted via acomputer system. The present inventions are also directed to suchsimulations and testing of the inventive device and/or circuitry, and/ortechniques implemented thereby, and, as such, are intended to fallwithin the scope of the present inventions. The computer-readable mediaand data corresponding to such simulations and/or testing tools are alsointended to fall within the scope of the present inventions.

It will be clear to one skilled in the art that many improvements andmodifications can be made to the foregoing exemplary embodiment withoutdeparting from the scope of the present disclosure.

What is claimed is:
 1. A photodetector device comprising: first andsecond light absorbing regions composed of semiconductor material, dopedrespectively n-type and p-type, the light absorbing regions beingconfigured to generate pairs of electrons and holes in response toabsorption of photons when light is incident on the device; first andsecond contact regions composed of semiconductor material and dopedrespectively p-type and n-type, wherein the contact regions have higherdoping concentrations than the light absorbing regions, and are labelledas p+ and n+ respectively; and first and second contacts connected tothe first and second contact regions respectively, wherein the n-typeand p-type light absorbing regions and the n+ and p+ contact regions arearranged in a sequence n+ p n p+ so that, after a voltage appliedbetween the n+ and p+ contacts is switched from a reverse bias to aforward bias, electrons and holes which are generated in the lightabsorbing regions in response to photon absorption drift towards the p+and n+ contact regions respectively, which causes current to start toflow between the contacts after a time delay which is inverselyproportional to the incident light intensity.
 2. The device of claim 1,wherein the first and second light absorbing regions are arrangedside-by-side in relation to a substrate.
 3. The device of claim 2,wherein the first and second contact regions are arranged laterallyspaced apart either side of the first and second light absorbingregions.
 4. The device of claim 1, wherein the first and second lightabsorbing regions are formed as respective epitaxial layers arranged ontop of one another in relation to a substrate.
 5. The device of claim 4,further comprising insulating trenches extending vertically through thelight absorbing regions and at least one of the contact regions so as tosubdivide the photodetector into an array of pixels that areindependently contacted.
 6. The device of claim 5, further comprising asemiconductor circuit layer arranged on the epitaxial layer that formsone of the first and second contact regions, the circuit layercomprising an array of read out sensors for the photodetector's pixelarray electrically connected to the pixels with vias.
 7. The device ofclaim 5, wherein each pixel is subdivided into an array of sub-pixels byfurther insulating trenches which for each pixel are disposed laterallyinside the pixel-defining insulating trenches and which extendvertically through one of the contact regions and at least one of thelight absorbing regions, but not as far as the other of the contactregions, so that the sub-pixels of any one pixel remain commonlycontacted.
 8. The device of claim 1, wherein the first and second lightabsorbing regions are arranged such that one of the first and secondlight absorbing regions is formed as an epitaxial layer on a substrate,or integrally with the substrate, and the other of the first and secondlight absorbing regions is formed as an embedded region within theepitaxial layer.
 9. The device of claim 1, wherein the first and secondlight absorbing regions are arranged such that one of the first andsecond light absorbing regions is formed in a first part as an epitaxiallayer on a substrate, or integrally with the substrate, and in a secondpart as an embedded region within the epitaxial layer or the substrate,and wherein the other of the first and second light absorbing regions isformed as a further embedded region within the epitaxial layer.
 10. Thedevice of claim 9, wherein the embedded region and the further embeddedregion are separated laterally by a portion of the epitaxial layer orthe substrate.
 11. The device of claim 9, wherein one of the first andsecond contact regions is formed as a still further embedded regionwithin the embedded region of the first or second light absorbingregions respectively
 12. The device of claim 9, wherein the first andsecond contact regions are formed as respective still further embeddedregions within the embedded regions of the first and second lightabsorbing regions respectively.
 13. The device of claim 1, wherein oneof the first and second contact regions is formed as an epitaxial layeron the substrate and at least one of the first and second lightabsorbing regions is formed at least in part as a further epitaxiallayer on the epitaxial layer of said one of the first and second contactregions.
 14. The device of claim 1, wherein at least one of the firstand second contact regions is formed as an embedded region within anepitaxial layer which forms at least a part of the first and secondlight absorbing regions respectively.
 15. The device of claim 1, whereinthe first and second contact regions are formed as respective laterallyspaced apart first and second embedded regions within an epitaxial layerwhich forms at least a part of the first and second light absorbingregions respectively.
 16. The device of claim 1, wherein one of thefirst and second contact regions is formed as laterally spaced apartfirst and second embedded regions formed within an epitaxial layer whichforms at least a part of one of the first and second light absorbingregions respectively.
 17. A method of manufacturing a photodetectordevice, the method comprising: fabricating first and second lightabsorbing regions using semiconductor material doped respectively n-typeand p-type, the light absorbing regions being configured to generatepairs of electrons and holes in response to absorption of photons whenlight is incident on the device; fabricating first and second contactregions composed of semiconductor material and which are dopedrespectively p-type and n-type, wherein the contact regions have higherdoping concentrations than the light absorbing regions, and are labelledas p+ and n+ respectively; and providing first and second contactsconnected to the first and second contact regions respectively, whereinthe n-type and p-type light absorbing regions and the n+ and p+ contactregions are arranged in a sequence n+ p n p+ so that, after a voltageapplied between the n+ and p+ contacts is switched from a reverse biasto a forward bias, electrons and holes which are generated in the lightabsorbing regions in response to photon absorption drift towards the p+and n+ contact regions respectively, which causes current to start toflow between the contacts after a time delay which is inverselyproportional to the incident light intensity.
 18. The method of claim17, wherein the first and second light absorbing regions are fabricatedas respective epitaxial layers arranged on top of one another inrelation to a substrate.
 19. The method of claim 18, further comprisingfabricating insulating trenches extending vertically through the lightabsorbing regions and at least one of the contact regions so as tosubdivide the photodetector into an array of pixels that areindependently contacted.
 20. A method of operating a photodetectordevice, the method comprising: providing a photodetector device with:first and second light absorbing regions composed of semiconductormaterial, doped respectively n-type and p-type, the light absorbingregions being configured to generate pairs of electrons and holes inresponse to absorption of photons when light is incident on the device;first and second contact regions composed of semiconductor material anddoped respectively p-type and n-type, wherein the contact regions havehigher doping concentrations than the light absorbing regions, and arelabelled as p+ and n+ respectively; and first and second contactsconnected to the first and second contact regions respectively, whereinthe n-type and p-type light absorbing regions and the n+ and p+ contactregions are arranged in a sequence n+ p n p+; and operating thephotodetector device by repeatedly: applying a voltage to reverse biasthe n+ and p+ contacts; switching the reverse bias voltage to a forwardbias voltage so that after said switching electrons and holes which aregenerated in the light absorbing regions in response to photonabsorption drift towards the p+ and n+ contact regions respectively; andsensing for onset of current flow between the first and second contactsand measuring a time delay between said switching and said onset,wherein the time delay is inversely proportional to the incident lightintensity.